The present invention relates to fusion bonding, particularly to vacuum fusion bonding, and more particularly to a process involving vacuum pull down that enables increasing the bonding temperature to produce enhanced bonding.
Typical bonding processes require essentially optically flat substrates in order to assure uniform sealing. This is particularly the case in large substrate applications (e.g., &gt;4-inch substrates). The primary problem in bonding substrates without voids is that most substrates are non-uniform in flatness. This results in non-contact areas when substrates are aligned for bonding. The non-contact areas cause several problems in that gas or air can be entrapped in the interstitial space where the substrates are not in contact. This will lead to a void and non-bonded area and also has the potential of forcing the substrates apart preventing any bonding and/or misaligning the substrates which might be registered on the other.
The present invention provides a bonding process which overcomes the above-mentioned problems. The process includes incorporating vacuum pull down to ensure uniform surface contact during the bonding process. The process provides for effectively bonding arbitrary sized and shaped substrates, such as glass, plastics, alloys, etc., which have a moderate melting point with a gradual softening point curve. The process is especially effective in bonding applications wherein one or both of the substrates to be bonded contain microchannels or grooves to be embedded between the substrates, and which can be used to apply vacuum between the substrates during the bonding process. The vacuum pull down, in addition to providing a differential pressure, enables both holding and alignment of the substrates.